1. Field of the Invention
The present invention relates to a semiconductor process and more particularly to a process for plating precious alloyed metals, such as AuSn, AuSnIn, AgSn, AuIn and AgIn, for use, for example, for attaching a semiconductor die to a substrate or housing in high reliability applications.
2. Description of the Prior Art
Conventional integrated circuits include a semiconductor die cut from a semiconductor wafer to a standard chip size. The semiconductor die is normally attached to a substrate or housing by way of an adhesive, such as an epoxy or solder. The adhesive is known to be cured at relatively high temperatures, such as 150xc2x0 C. to 160xc2x0 C. Unfortunately, such a technique is known to produce air pockets between the semiconductor die and the substrate that can cause a void therebetween.
There are other risks associated with the use of epoxy for attaching a semiconductor die to a substrate. For example, since epoxy can wet virtually any material, in some situations, the epoxy has been known to wet from the edge of the semiconductor die to the top causing a short circuit.
In order to avoid these problems in relatively high reliability applications, such as military and space applications, the semiconductor die is known to be attached to the substrate with precious alloyed metals, such as gold-tin (AuSn), silver-tin (AgSn), gold-indium (AuIn) and silver-indium (AgIn). However, because of the largely differential melting point of the metal components in the alloys, such alloys are not suitable for evaporation and heavily waste the alloyed metal during sputtering processes in order to obtain a thickness of the solder alloys from 5 xcexcm to 25 xcexcm.
As such, precious alloyed metal solder techniques are known to have been developed. These techniques are known to be rather complicated and expensive. For example, in one known process, precious alloyed metal preforms are used. Such precious metal alloyed preforms are known to be produced by a rather complicated metallurgical process and are thus expensive. An example of such a precious alloyed metal preform is disclosed in U.S. Pat. No. 5,427,865, hereby incorporated by reference. As disclosed in U.S. Pat. No. 5,234,865, solder preforms, such as precious alloyed metal solder preforms, are disposed between two components to be soldered, such as a semiconductor die and a substrate. The assembly is heated to a temperature greater than the melting point of the precious metal alloy, which causes the solder to reflow and, upon cooling, attaches the wetable surfaces of the semiconductor die to the substrate.
Because of cost of producing precious alloyed metal solder preforms, other techniques have been developed. For example, gold-tin alloyed solder is known to be formed by depositing layers of gold/tin/gold onto a substrate by vacuum deposition. These layers are then alloyed together at a relatively high temperature, for example, 220xc2x0 C., for at least three (3) hours to allow the gold and tin layers to inter-diffuse and form a gold-tin alloy. Although such a process is effective for forming a gold-tin alloy, the process is extremely expensive and requires a relatively large capital equipment investment as well as involves a relatively high labor cost. Moreover, the semiconductor dies are known to be temporarily attached to the substrates by way of wax or a thermal film. Due to the exposure to relatively high temperatures for a relatively long period of time, such a process may result in contamination of the semiconductor die from the wax. Accordingly, there is a need for a process for attaching a semiconductor die to a substrate with a precious alloyed metal that is relatively simpler and less expensive than known processes and does not pose a contamination or short circuit risk to the semiconductor die.
The present invention relates to a relatively simple and inexpensive process for plating precious alloyed solder, such as AuSn, AuSnIn, AgSn, AuIn and AgIn. Anodes are formed from pure metals in the alloy composition and disposed in a conducting solution. The target is also disposed in the conducting solution. The mass of the individual metal component in the alloyed solder that is transferred from the anodes is determined by Faraday""s law. Plating current is independently applied to each anode. The plating is conducted under an ultraviolet light sources to optimize the process. The plating process can be used to produce different alloyed solder compositions for various applications including attaching a semiconductor die to a substrate. Since the process does not involve exposure of the semiconductor die to a relatively high temperature for a relatively long time, the process does not pose a risk of contamination of the semiconductor devices by the adhesive or wax used to hold the die in place on the substrate during processing. Moreover, unlike earlier known processes which utilize epoxy, the precious alloyed solder do not wet the entire die but only the metal contact areas, thus avoiding potential short circuit to the die.